Solar cells are providing widespread benefits to society by converting essentially unlimited amounts of solar energy into useable electrical power. As their use increases, certain economic factors become important, such as high-volume manufacturing and efficiency.
With reference to the schematic views of exemplary solar cells of, e.g., FIGS. 1-2 below, solar radiation is assumed to preferentially illuminate one surface of a solar cell, usually referred to as the front side. In order to achieve a high energy conversion efficiency of incident photons into electric energy, an efficient absorption of photons within a silicon wafer is important. This can be achieved by a low parasitic optical absorption of photons within all layers except the wafer itself. Surface texturization is a well known technique for improved capture of the incident light radiation. Texturization may be effected in several ways, but most commonly formed using a wet acid or alkaline etch, in which the surface of the wafer etches non-uniformly, leaving a dense field of pyramids or conical-spikes over the entire surface of the solar cell substrate. However, it is understood that the geometrical shape and/or surfaces may be textured in any shape beneficial for improved solar cell efficiency.
An important parameter for high solar cell efficiency is surface passivation. Surface passivation is generally considered to effect the suppression of recombination of electrons and holes at or in the vicinity of certain physical surfaces of the wafer. Surface recombination can be reduced by the application of dielectric layers over the substrate. These layers reduce the interface density of states and therefore reduce the number of recombination centers. The most prominent examples are thermally grown silicon oxide and PECVD deposited silicon nitride. Other examples of surface passivating layers include intrinsic amorphous silicon, aluminum nitride, aluminum oxide, etc. This principle is illustrated in FIG. 1. The aforementioned layers can also provide an electrical charge which introduces a repelling force, which reduces the availability of carriers of the opposite polarity to recombine, thereby reducing recombination rates. The most prominent examples of charge carrying passivating layers are silicon nitride and aluminum oxide. Another method of reducing the amount of carriers of one type close to the surface is the diffusion of doping atoms either of the same or the opposite doping of the wafer doping type. In this case doping levels in excess of the wafer doping are necessary to obtain a high-low junction (also commonly called back surface field or front surface field) or a p-n junction. This can be combined with other methods of surface passivation mentioned above.
High efficiency solar cells require good surface passivation combined with a technique to make electrical contact to the substrate with minimal recombination losses. Exemplary solar cell structures and practical methods of forming the same, addressing the issues above, are the subject of this invention.